Current developments in medical engineering lead to an ever-increasing number of medical devices that are capable of generating alarm. As a result, the avoidance of clinically irrelevant alarms is also of ever-increasing significance. Clinically irrelevant alarms triggered too frequently are among the most dangerous technical risk factors in hospitals. The hazard potentials are numerous and varied. Alarm limits could be adjusted by the clinical staff to reduce the noise level outside their reasonable range and they may become ineffective as a result, or the acoustic volume of the alarm may be set at a barely perceptible minimum. The frequency of alarms could lead to an insidious desensitization of the clinical staff, so that there is a risk that the staff will not respond adequately to an actual hazard to the patient any more. Excessively frequent and medically irrelevant alarms may lead, furthermore, to annoyance, frustration and stress, which may in turn lead as a consequence to declining concentration as well as an impairment of the work performance and performance capacity. In addition, the work process of the clinical staff could be disturbed or interrupted, so that important activities, e.g., the medication of a patient, may be temporarily forgotten, or errors may be creep up on resumption.
On the other hand, a smaller number of alarms increases at the same time the attentiveness to the now prominently relevant alarms and markedly increases the patient's safety at the same time. Furthermore, the care becomes more efficient, because it can be carried out trouble-free and, in addition, more time is available at bedside. Fewer alarms also mean a reduction of the stress level, more restful sleep and faster recovery for the patient, which results in a shorter hospital stay.
As long as there is no contraindication and additional clinical parameters that are taken into consideration, for example, the oxygen saturation or the CO2 partial pressure, are within an acceptable range during the monitoring of the respiratory activity, immediate alarming is dispensable, at least if the limit values are transgressed only briefly and/or slightly.
Methods and devices for avoiding clinically irrelevant alarms, in which the distinction between clinically relevant and clinically irrelevant alarms is performed on the basis of the minute volume, are known for this from the state of the art. The minute volume is used as a standard in medicine. In mandatory ventilation, the minute volume corresponds to the product of the tidal volume applied by the respiration rate with the unit liters per minute. Thus, a minute volume of 9 L/minute is obtained, for example, from a set tidal volume of 600 mL and a respiration rate of 15 per minute. An (expiratory) time volume corresponds in this case to the quantity of the (exhaled) air flow over a defined time period.
Since the minute volume is an important indicator for the ventilation of the patient, monitoring of the value by ventilators used in intensive care is obligatory. If the resulting minute volume is constant and hence available at any time during mandatory ventilation, an exact result can be calculated in case of spontaneous breathing, purely theoretically, only by integrating the last flow value at the end of the time interval of one minute. However, this may also take place over shorter time intervals and extrapolated to one minute. Since the result reflects both breathing efforts made by the patient immediately before and those that had occurred already a minute ago, the current respiratory activity and possible hazards, e.g., hypopnea, can be inferred from a minute volume calculated in this way only conditionally.
Consequently, the calculated minute volume should generally be, on the one hand, an indicator of the current respiratory activity and thus fast enough for changes to be reflected in the value within a short time. On the other hand, the minute volume should be slow enough for individual breaths to affect the value only slightly and consequently for the time course of the minute volume to have only a slight “waviness.” Based on the different lung constants of adults, children and new and premature babies, the concrete design of the filter used for the calculation is adapted, as a rule, to the particular patient categories. Prior-art filters generate as a signal a step response. The time characteristics of the output signal of a transmission member, for example, of the filter, in case of a change in the signal at the input is described as a step response, so that the filter characteristic can be unambiguously characterized on the basis of this step response. A time value T90, which is linked with this, indicates the time after which 90% of the maximum of the output value was reached. The T90 value is correspondingly an indicator of the delay with which the input signal affects the output signal. A shortening of the T90 time is associated, in general, with increased oscillations on the output signal.
The signals generated by are usually processed in conventional medical devices in a monitoring device. Such monitoring devices have, in turn, a monitoring range, which can be divided, in principle, into at least two ranges: An alarm-free range (desired range) and into one or more ranges in which an alarm is mandatory. Both arise directly from explicitly settable or derived alarm limits. The monitoring of a measured value, for example, a minute volume, leads, as a rule, directly to an alarm as soon as a measured value exceeds or drops below a limit value, i.e., the measured value moves from the alarm-free range into the alarm-requiring range (cf. FIG. 1).
In conventional alarm methods and devices, the minute volume is monitored on the basis of exactly such a lower limit and upper limit, which can be set and adjusted by the user at any time. The alarm limits are set individually for each patient and depend especially on his physical constitution and medical condition. A corresponding alarm is triggered and produced by the device acoustically as well as optically exactly as soon as the current value of the minute volume is below the set lower alarm limit (MV-low) or above the set upper alarm limit (MV-high). In clinical practice, an alarm limit is occasionally transgressed only briefly and/or only very slightly. This is due, for example, to the deterioration of the physiological regulation of respiration or to movement of the patient. The latter may occasionally lead to a short-term slippage of the breathing mask, so that the exhaled lung volume is not detected by the ventilator and the determined minute volume is thus smaller than it actually is.
By setting an alarm delay (MVdelay), it is possible in such case that the alarm will not be triggered immediately after the measured value drops below MVlow. Alarms based on brief transgressions of the limits can thus be effectively suppressed. With the aim of avoiding false alarms occurring during the monitoring and of reducing the number of alarms hereby, delay times (MVdelay) set as fixed values or delay times that can be set by the user can already be set on some ventilators to briefly suppress alarms. However, it is problematic in connection with the setting of such delay times that the particular course of the values of the minute volume cannot be accessed. Depending on the patient's condition, clinical picture, individual constitution, etc., and on the extent by which the measured value exceeds or drops below the limit value, delay times of different duration are needed in order to effectively rule out a risk, on the one hand, but not to generate a needlessly early alarm, on the other hand.